1. Field of The Invention
The present invention relates to a system and method for controlling an idling speed for an internal combustion engine linked to, for example, a steel belt type electro-continuously variable transmission.
2. Description of The Background Art
A Japanese Patent Application First Publication Showa 63-38747 published on Feb. 19, 1988 exemplifies a steel belt type electro-continuously variable transmission (non-stage power transmission, or abbreviated as ECVT) for an automotive vehicle.
In the electro-continuously variable transmission, a so-called torque converter which transmits output power of an associated engine to its output shaft via a fluid is installed and, in addition to this, a steel belt is wound around two pulleys so as to provide the transmission means for transmitting the output power of the engine to the output shaft.
In detail, the continuously variable transmission includes: an electromagnetic clutch mechanism for starting the vehicle which intermittently connects or disconnects a crankshaft of the engine to an input shaft thereof; a synchro-mesh type forward/rearward switching mechanism; the steel belt wround around a primary pulley linked to the forward/rearward switching mechanism and a secondary pulley; a hydraulic pressure control valve mechacism which controls a hydraulic pressure in each pulley pressurized and supplied from an oil pump; and final gear ratio mechanism.
The oil pump is constituted by an outer tooth type involute gear pump directly driven by means of the engine crankshaft. Its drawn pressure is supplied to the primary pulley and secondary pulley via a hydraulic pressure control valve mechanism so as to be used as a working pressure.
That is to say, the hydraulic pressure control valve mechanism serves to control a flow in and out of a line pressure to and from the primary pulley and secondary pulley, an accelerator pedal depression quantity and engine revolution speed as signal sources and position each movable sheep of the primary and secondary pulleys to control a pulley ratio (gear ratio).
When a vehicle driver depresses the accelerator pedal and the gear ratio is placed in a first speed range, a predetermine line pressure is introduced to the primary gear. At this time, it is defined as a gear ratio change start point. At the gear ratio change start point, from where of the engine revolution speeds (required engine revolution speed) the gear range shift position is started is determined according to the magnitide of the accelerator pedal depression. During a steady state driving, as the requested engine revolution speed is increased with the gradual increase in the depression displacement on the accelerator pedal, the transmission shifts up toward second gear range, third gear range, and so on. On the other hand, when the driver releases the accelerator pedal during an overdrive mode, the gear ratio is shfted down to the first gear along with the decrease in the requested engine revolution speed.
In the electro-continuously variable transmission described above, the gradual downshift is carried out as the gradual reduction in the engine revolution speed and the downshift to the first speed range at the time of a predetermined low speed engine revolution in a case where a normal braking operation is carried out during a steady state drive over 2-nd gear range or acceleration drive.
In a case, however, where an abrupt deceleration due to an abrupt braking operation is carried out, the engine revolution speed abruptly drops below 1,500 r.p.m. along with an interruption of the electromagnetic clutch mechanism. Therefore, since an revolution speed of the oil pump also abruptly drops accordingly, the drawn pressure required to change from third or second gear range to the first range cannot be maintained any more.
Consequently, even if the immediate acceleration is tried after the abrupt deceleration, the gear range is not yet placed in the first speed range. Then, a time lag occurs and acceleration feeling is remarkably reduced.